Alkyl halides, redox potentials

The redox pair formed from oxidizing the zero-valent iron has a reduction potential of -0.440 V therefore, zero-valent iron can reduce hydrogen ions, carbonate, sulfate, nitrate, and oxygen, in addition to alkyl halides (Matheson and Tratnyek, 1994). Both Equation (13.2) and Equation (13.3) cause the pH... 

TABLE 12.1 Redox Potentials for Alkyl Halides (RX) and Aryl Chlorides (ArClx) in Dimethyl Formamide at a Glassy-Carbon Electrode"... 

Oxidation-reduction (redox) reactions, along with hydrolysis and acid-base reactions, account for the vast majority of chemical reactions that occur in aquatic environmental systems. Factors that affect redox kinetics include environmental redox conditions, ionic strength, pH-value, temperature, speciation, and sorption (Tratnyek and Macalady, 2000). Sediment and particulate matter in water bodies may influence greatly the efficacy of abiotic transformations by altering the truly dissolved (i.e., non-sorbed) fraction of the compounds — the only fraction available for reactions (Weber and Wolfe, 1987). Among the possible abiotic transformation pathways, hydrolysis has received the most attention, though only some compound classes are potentially hydrolyzable (e.g., alkyl halides, amides, amines, carbamates, esters, epoxides, and nitriles [Harris, 1990 Peijnenburg, 1991]). Current efforts to incorporate reaction kinetics and pathways for reductive transformations into environmental exposure models are due to the fact that many of them result in reaction products that may be of more concern than the parent compounds (Tratnyek et al., 2003). 

Copper(I) complexes catalyse a variety of organic reactions which are of synthetic and industrial importance.305 In such processes that involve halide abstraction from aryl or alkyl halides, the abstraction step by a Cu(I) catalyst is believed to be the rate-determining step. In order to circumvent the property of facile disproportionation of Cu, various methods of stabilising Cu(I) and influencing reaction rates were considered.306 A kinetics study of ligand (L) effects on the reactivity of Cu(I)L complexes towards C13CC02 was undertaken. The results indicated that the rate of the chlorine abstraction reaction was affected by several factors. These were the redox potential of the Cu(II/I)L couple, the hybridisation on Cu(I) in the Cu(I)L complex, steric hindrance, and electron density on the central Cu(I) cation at the binding site of the chlorine atom to be abstracted. The volume of activation,... 

This mechanism is satisfying but not universal, as has been hinted at above. For example an alternative initiation step was discovered for the reaction of [M(SCN)(N2)(dppe)2] with Bu"I. The reaction was first-order in each reactant and the product, [M(SCN)(N2Bu")(dppe)2] " did not include the halogen from the alkyl halide. The mechanism shown in (18) was favoured on the grounds that the redox potential of [M(SCN)(N2)(dppe)2] is about a volt more negative than that of [M(N2)2(dppe)2], so the anion is more likely to transfer an electron to the halide. Moreover, the electron transfer step would generate an M species plus a radical, which is exactly what results from metal-assisted homolysis of the alkyl halide. 

The arguments given state that easily oxidized bases will be good nucleophiles toward alkyl halides and also strong bases toward the proton. This statement means that E° and H in the Edwards equation are no longer independent parameters but essentially one and the same property for transition metal bases. For bases where the donor atom is a nonmetal, this correlation is normally not the case. The fluoride ion is a stronger base than the iodide ion but is more difficult to oxidize. Still, examination of bases of the representative elements more closely to see if E° and H are truly independent is worthwhile. Actually E°, the redox potential in water, is not the best parameter to use. E° is measurable only for a few nucleophiles and is complicated by the nature of the products formed. For example, in... 

Primary or secondary alkyl halides RX are completely transformed into alkane RH after reflux for several hours in 0.1 M THF solution of Sml (one equivalent). There is no reaction at room temperature. As predicted from redox potentials the reactivity decreases in the following direction RI > RBr > RCl. Phenyl or vinyl halides are unreactive, apart from 1-io-do naphthalene which was reduced to naphthalene (21), presumably because of the decrease in redox potential by the naphthalene ring. Allylic and benzylic halides are very reactive and give a coupling reaction in a few minutes at room temperature (eq. 9]). 

A parallel development was initiated by the first publications from Sawamoto and Matyjaszweski. They reported independently on the transition-metal-catalyzed polymerization of various vinyl monomers (14,15). The technique, which was termed atom transfer radical polymerization (ATRP), uses an activated alkyl halide as initiator, and a transition-metal complex in its lower oxidation state as the catalyst. Similar to the nitroxide-mediated polymerization, ATRP is based on the reversible termination of growing radicals. ATRP was developed as an extension of atom transfer radical addition (ATRA), the so-called Kharasch reaction (16). ATRP turned out to be a versatile technique for the controlled polymerization of styrene derivatives, acrylates, methacrylates, etc. Because of the use of activated alkyl halides as initiators, the introduction of functional endgroups in the polymer chain turned out to be easy (17-22). Although many different transition metals have been used in ATRP, by far the most frequently used metal is copper. Nitrogen-based ligands, eg substituted bipyridines (14), alkyl pyridinimine (Schiff s base) (23), and multidentate tertiary alkyl amines (24), are used to solubilize the metal salt and to adjust its redox potential in order to match the requirements for an ATRP catalyst. In conjunction with copper, the most powerful ligand at present is probably tris[2-(dimethylamino)ethyl)]amine (Mee-TREN) (25). 

Halogen atom transfer reactions are relatively uncommon for alkyl halides and Ru(bpj)3. This lack of reactivity is primarily due to the excited state Ru(bpj)3 being an outer-sphere redox reagent, and for alkyl halides both the oxidation and reduction potentials have values that make such electron transfer reactions unfavorable. These considerations are particularly valid for chlorocarbons, but for bromocarbons or iodocarbons it is possible that selective photoreactions with Ru(bpj)3 may be observed. 

Efficient electrocatalysis can be exemplified by dehalogenation of ai and alkyl halides (ArX). The direct electrochemical r uction of these compounds requires very low redox potentials (-1.6 V). However, in the presoice of the zero-valence nickel tetraphosphine complex, efficirat ddialogenation via the scheme... 

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